Users of computing devices (e.g., laptops, cellular phones, and personal digital assistants) often need to communicate in real time. A common form of real-time communications is provided by instant messaging services. An instant messaging service allows participants at endpoints to send messages and have them received within a second or two by the other participants in a conversation. The receiving participants can then send responsive messages to the other participants in a similar manner. To be effective, a real-time conversation relies on the participants' becoming aware of, reviewing, and responding to received messages very quickly. This quick response is in contrast to conventional electronic mail systems in which the recipients of electronic mail messages respond to messages at their convenience.
When an initiating participant wants to start a real-time conversation, that participant needs to know whether the intended participants are available to respond in real time to a message. If not, then communication via conventional electronic mail, voice mail, or some other mechanism may be more appropriate. For example, if the computers of the intended participants are currently powered off, then a real-time conversation may not be possible. Moreover, if their computers are currently powered on, but the intended participants are away from their computers, a real-time conversation is also not possible. The initiating participant would like to know the availability of the intended participants so that an appropriate decision on the form of communication can be made.
Presence services provide this availability information. The availability status of an entity such as a computer system or a user associated with that computer system is referred to as “presence information.” Presence information identifies the current “presence state” of the user. Users make their presence information available to a presence server so that other users can decide how best to communicate with them. For example, the presence information may indicate whether a user is logged on (“online”) with an instant messaging server or is logged off (“offline”). Presence information may also provide more detailed information about the availability of the user. For example, even though a user is online, that user may be away from their computer in a meeting. In such a case, the presence state may indicate “online” and “in a meeting.”
A presence service may provide a presence server for distributing presence information. A publishing user (“publisher”) may run client software that provides their presence information to a presence server that then provides the presence information to subscribing users (“subscribers”). Thus, a presence server may use a subscriber/publisher model to provide the presence information for the users of the presence service. Whenever the presence information of a publishing user changes, the presence server is notified of the change by that user's client software and in turn notifies the subscribing users of the change. A subscribing user can then decide whether to initiate a real-time communication and the type to initiate based on the presence information of the intended participants. For example, if the presence information indicates that a publishing user is currently on a conference telephone call, then the subscribing user may decide to send an instant message, rather than place a telephone call, to the publishing user. RFC 2778, entitled “A Model for Presence and Instant Messaging,” is a specification relating to presence information in instant messaging systems. RFC 3856, entitled “A Presence Event Package for the Session Initiation Protocol,” is a specification relating to presence information using the Session Initiation Protocol (“SIP”).
Current presence services support the publication of increasingly larger numbers of presence states by publishers. Current presence services also provide publishers increased control over how others, such as the subscribers, see their presence information, and how the presence servers interact with the subscribers. For example, publishers are able to control the publication of their presence information on a per subscriber basis. Unfortunately, the increasing numbers of options (e.g., supported presence states, increased control, etc.) that can be set for each of the subscribers result in increasing complexity that is daunting even to advanced users of the presence services.